This invention relates to steel product having a coating based on a hypereutectic zinc-aluminium alloy which may be made by continuous dip coating of a steel strip.
Continuous dip coating of a steel strip is a technique which is known and has been extensively applied for many years. Basically, it consists of passing a steel strip through a bath of molten zinc or zinc alloy then solidifying the coating after having regulated its thickness.
In accordance with this technique, it is normal practice to use, in particular, zinc-aluminium alloys. It is known that these alloys have a eutectic which is in the proportion of approximately 5% by weight of aluminium. A hypereutectic zinc-aluminium alloy is therefore a zinc-aluminium alloy containing at least 5% by weight of aluminium.
This invention relates to the deposition of a coating based on a hypereutectic zinc-aluminium alloy and, more particularly, comprising an alloy which contains, typically, by weight, in addition to the zinc, 55% of aluminium and 1.6% of silicon. These alloys combine the high resistance to corrosion of the aluminium and the cathodic protection provided by the zinc. The purpose of adding silicon is to modify the reaction between the iron in the steel strip and the aluminium in the coating. In the absence of silicon, this reaction results in a very considerable loss of iron and a coating which is entirely transformed into Fe--Al which has no adherence or ductility.
It is however apparent that this coating, as known, presents serious defects affecting the adherence and ductility when it is subjected to bending or forming, as is frequently necessary in the case of panels intended, in particular, for manufacturing purposes. These defects cause the coating to crack and the cracks formed even spalling. This brittleness and lack of adherence of the coatings, as known, appears to be the result of three principal causes. Firstly, the coating comprises a two phase metastable mixture which does not solidify simultaneously. This results in the appearance of a structure which comprises zones rich in zinc and zones rich in aluminium, which have different physical properties generating internal stresses. Also, at the interface between the steel substrate and the zinc-aluminium coating, a layer of brittle intermetallic particles of Fe--Al--Zn--Si type is formed. Finally, the silicon added to modify the reaction between the iron and the aluminium does not remain entirely in solution. On cooling, it is precipitated in the form of needles which are the origin of stress concentrations and result in the brittle nature of the coating.
An attempt has already been made to remedy these disadvantages by means of specific heat treatments. In particular, it has been proposed to heat the coating to 300.degree.-350.degree. C. for three minutes or, again, to carry out an annealing stage at 150.degree. C. for a period of twenty-four hours. These treatments have been found to be technically satisfactory but are not viable economically because of the resulting costs.